This invention relates to cardiac pacemakers and, more particularly, to dual chamber cardiac pacemakers adapted to be operated in a plurality of operating modes, including a mode which incorporates P wave synchronous operation, e.g. VDD, DDD.
The advantages of cardiac pacing in different modes, selected to different patient conditions, is now well recognized in the art. Early pacer systems were solely ventricular, which was sufficient for management of patients with complete heart block and Stokes-Adams attacks. However, ventricular pacemakers, even when demand pacemakers, are not equipped to take advantage of atrial activity, and thus are limited in their efficiency. Subsequently, artrial synchronous, ventricular pacemakers were introduced, having a lead for sensing P signals from the atrium and another for pacing the ventricle after a suitable P-R (A-V) interval. Such a pacemaker, e.g. VDI or VDD, allows the atrium to control the heart's response rate, the ventricle being paced at the atrial rate up to a predetermined upper rate limit. Such synchronous pacers have incorporated means for pacing the ventricle at a 1:2 rate relative to the atrium, or even higher ratio, when the sensed atrial rate exceeds the predetermined maximum rate.
Another form of A-V, or dual chamber pacer that has been utilized is the sequential pacemaker (DVI), which paces both the atrium and the ventricle with an appropriate A-V delay which is timed by the pacemaker. Other pacing modes have been developed, and are classified in accordance with the commonly adopted ICHD code system. In this classification system, the first letter represents the chamber(s) paced (A for atrium; V for ventricle; D for dual), the second letter represents the chamber(s) sensed, and the third letter represents the sensing function, i.e. inhibited (I), trigger (T), dual (D). Known pacing modes include AAI, AAT, VAT, VDD, VDI, VVI, VDD, DVI, VVI, DDT, and DDD. Other codes are used to represent programmability and means for dealing with tachycardia.
With the advent of programmability of pacemakers, as well as improved lead systems for transmitting electrical signals to and from the atrium as well as the ventricle, dual chamber modes of operation are becoming more common and are expected to receive increased use. The advantages of the DDD, or universal pacer, are being more widely considered. In plural operating mode systems, which include P wave (or atrial) synchronous operation, it has been known that pacemaker induced, or mediated tachycardia can be a problem. The problem is caused by the retrograde transmission from the ventricle to the atrium of an electrical signal due to a delivered ventricular stimulus pulse, thereby inducing a retrograde P wave. If the pacemaker is permitted to carry on over too great a number of cycles, wherein retrograde P waves follow delivered stimulus pulses, a dangerous pacemaker mediated, i.e. supported, tachycardia results, which should somehow be controlled.
Another problem that exists in operating in any mode which involves P wave synchronous operation originates from attempts to maintain ventricular pacing at a rate no greater than a predetermined maximum rate, even when the natural atrial signal is above such rate. One prior art means of achieving this is to effectively extend the A-V interval for a number of pacer cycles, so that the pacing rate does not exceed the predetermined maximum during those cycles, and then occasionally skip a ventricular pulse and come back into normal synchronous operation. However, this means of operation carries the risk of excessively long actual A-V times, which can have a detrimental effect, including establishing conditions favorable to pacemaker mediated tachycardia.